This invention relates to a scan-lock control for use with radio receivers. The invention more particularly relates to a scan-lock control device for use with solid state, digitally synthesized receivers. Specifically the present invention incorporates a chassis, separate from the receiver to be scanned, having scanning and lock-up circuits with a power supply therefor, a group of plug-in circuit cards which may be plugged into the chassis and each having one of the frequency channels to be scanned programmed thereon, and an interface component interconnected with or physically mounted within the receiver for interconnecting the scan-lock control and receiver in such a way as to add the frequency scanning and lock-up functions without affecting any of the receiver's other locally controlled functions.
Operational requirements exist to scan a list of high frequency (HF) radio frequencies. One example is the "Digital Selective Call Device" under development by GTE Sylvania. Another is the existing Bell System two-tone selective ringer. Unattended radioteletype reception is still another application where automatic acquisition of the optimum HF frequency is required. The requirement exists at both ship stations and coast stations.
The requirement for HF multi-frequency reception has been traditionally met in the prior art by a number of single frequency receivers equal to the number of frequencies to be received. This is typically one frequency in each of the six marine HF bands for each station being watched. If more than one station is to be watched, the number of receivers becomes prohibitive.
If automatic reception of teletype or selective ringing on single side band is being employed, the receivers must be quite stable, and therefore quite expensive.
An emerging requirement in ships for unattended reception of selective call signals and direct printing signals has given rise to the need for reception on several radio frequencies, either simultaneously or sequentially. It is precisely this requirement which the scan-lock control device of the present invention is intended to satisfy. The recent appearance in ships of digitally synthesized receivers has made it possible to satisfy this need both economically and reliably.
In the shipboard application, the output of the synthesized radio receiver is fed to a selective call device. This device may be an existing Bell System two-tone selective ringer, common on many ships, or the digital selective caller under development by GTE Sylvania.
Still another application on shipboard is the unattended reception of direct printing signals, using the teletype stunt box as a selective call selector. A similar application is particularly common in naval ships which receive direct printing broadcast more or less continuously.
In the case of either type of selective ringer or of the direct printing terminal, the coast station or stations from which the ship might expect to receive calls or signals, transmits on a family of frequencies. These are typically one frequency each in the 4, 6, 8, 12, 16 and 22 MHz marine bands. One or more of these frequencies can be expected to reach the desired ship at the time of day and at the distance between the coast and the ship stations.
The frequency which will best reach the ship which it is intended to selectively call or communicate with, will change on a daily basis as the ship steams from place to place, and on an hourly basis as the sun's position around the earth affects ionospheric propagation of radio waves.
It is therefore unreasonable to set a radio receiver to a single frequency and expect it to receive signals from a distant coast station for any length of time. As the distance, but more important, the time of day changes, the optimum frequency will change. When the selective call device or direct printer is placed on watch, the frequency to which the receiver is initially set will receive signals at the time, but after a period ranging from a few minutes to a few hours, the frequency will fade. Unless the receiver is returned to that frequency in the family of frequencies transmitted by the coast station which most closely approximates the optimum frequency, satisfactory signals will no longer be received and the selective call device to which the receiver is connected will be, for all practical purposes, inoperative.
Certainly, a single frequency cannot be expected to continue to receive satisfactory signals from a distant coast station during the twelve-hour period that ship's radio officers are normally "off watch."
The problem of changing propagation and changing optimum frequency has traditionally been solved by fitting a number of receivers equal to the number of frequencies in the family of frequencies employed by the coast station. This is typically five or six frequencies.
If a number of receivers, each tuned to one of the family of frequencies is employed, the cost gets multiplied five or six times. If more than one coast station is to be watched by the ship, each with a family of five or six frequencies, the cost becomes prohibitive. Together with increased cost comes an increase in the failure rate as the amount of equipment in the system increases.
Moreover, the receivers employed for selective calling or for direct printing must be of an uncommonly high order of frequency accuracy and stability. GTE Sylvania reports that differences in the frequency of the ship's receiver and the frequency of the distant coast station transmitter as small as five Hertz results in a measurable increase in the data error rate. This agrees with direct printing experience wherein it is endeavored to maintain ship station transmitter frequencies within five Hertz. Those knowledgeable in the field agree that the data error rate mounts to an unacceptable magnitude when the frequencies differ by more than twenty Hertz. Radio receivers of such frequency accuracy and stability are especially expensive.
A simpler solution which forms the crux of the present invention and which is at once more economical and more reliable is a novel scan-lock receiver. A number of solid state digitally synthesized receivers have made their appearance on board ships in recent years. Under the present invention, these can now be processor controlled to scan a list of frequencies, stopping and locking onto the frequency on which satisfactory selective call or direct printing signals are received. Some receivers may also have to be frequency stabilized using an external standard.
The scan-lock control of the present invention is designed to determine whether or not a selective call signal or direct printing signal is present and to stop the scan and lock onto the frequency when, but only when, these signals are present. Happily, the GTE Sylvania selective call device and direct printing both accept audio frequencies of .+-.Hz shift centered on 1,500 Hz. This permits detection of the discrete audio tones 1,415 Hz and 1,585 Hz for both GTE Sylvania selective calling and for direct printing. The direct printing may employ Simplex TOR or CODEX error correction systems without affecting the scan-lock's ability to detect tones and lock onto the frequency. Of course, unprotected direct printing signals will also lock up the scan-lock control.
The Bell System two-tone selective ringer employs the two audio tones; 600 Hz and 1,500 Hz, alternated at 10 Hz.
The scan-lock device has therefore been designed in one embodiment to lock up on either tone pair; i.e. GTE Sylvania and direct printing at 1,415 Hz and 1,585 Hz, and Bell System at 600 Hz and 1,500 Hz.
Preferably, single side band voice, AM carrier, or CW will not lock up the scan-lock device. Therefore, normal radio traffic on the frequencies being scanned are ignored by the scanlock control, but selective call and direct printing signals are detected and locked onto.
The output of the radio receiver is connected to one or more of the selective call and direct printing components, and once the scan-lock control has stopped the frequency scan on a frequency which provides satisfactory signals, these components determine whether the particular ship is being selective called or not. If the selective call is for the ship, the alerting devices associated with the selective call decoder summons the radio officer or turns on equipment to receive a message to be read by him when he next comes on watch.
If the selective call is not for the ship, the call decoder will remain quiescent. Upon removal of the selective call signal, the scan-lock control resumes its search for a satisfactory frequency.
A digitally synthesized solid state receiver in combination with the scan-lock control device of the present invention provides the highest probability of receiving selective calls directed to the ship at the lowest cost and with the greatest equipment reliability.
One of the primary objects of the present invention is to provide a scan-lock control for a synthesized receiver of adequate stability. A synthesizer's ability to switch rapidly from one frequency to another, coupled with the relative ease of achieving high frequency stability, makes it well suited for this application.
Another object of the present invention is to provide a scan-lock control device for existing models of marine solid state digitally synthesized receivers. Under the present invention, a list of up to twenty discrete and field-programmable frequencies can be scanned at a rapid rate.
A further object and advantage of the invention is that lock-on frequency is achieved only when one or more of four discrete ringer or teletype tones is received.
Other objects and advantages of the present invention, in addition to those specifically mentioned above, will become readily apparent in the course of the following specification.